Supernova Remnants

The unprecedented broad bandwidth and excellent spectral resolution of
ASCA has had a major impact on all areas of research into the nature
of supernova remnants (SNRs). Recent results from ASCA have
increased the number of neutron star/SNR associations, debunking
the decade-long mystery of the paucity of such associations.
Additionally, ASCA has uncovered localized regions of non-thermal
X-ray emission inside of SNRs which are not associated with the
synchrotron nebulae but are produced by previously unrecognized
mechanisms. ASCA maps of SNRs in prominent X-ray emission lines and
selected continuum bands show variation in temperature, ionization,
chemical composition, and, indeed, even the nature of the underlying
emission mechanism. ASCA's increased sensitivity has allowed for a
systematic study of the remnants in the LMC leading to the discovery
of new ejecta-dominated remnants and an independent measurement of the
gas-phase abundances of the LMC. Highly absorbed Galactic SNRs, which
were weak and nondescript in previous soft X-ray observations, turn
out to be remarkable objects with booming emission lines when observed
using ASCA. These and other discoveries are leading to new
insights into the nature of the ejecta of young remnants, the physics
of supernova-induced shock waves, and the discovery and study of
pulsar-powered synchrotron nebulae.

The Connection between Neutron Stars and Supernova Remnants

Neutron Stars (NS) are thought to originate in SN explosions, however
the relatively few known associations of NS with SNR had led some to
doubt the underlying ideas of NS formation. ASCA has dramatically
improved this situation by increasing the number of known
associations. Among the current generation of X-ray instruments, only
the ASCA instruments possess the capability to separate the soft,
mostly thermal emission from the interstellar material swept up by the
SN shock from the hard, non-thermal emission from the synchrotron
nebula around a pulsar. If the compact object cannot be detected
directly, ASCA allows us to infer its existence by detecting its
interaction with the surrounding medium. Since this emission is not
subject to the beaming effects of the pulsar emission, it is
significantly more likely to be detected.

Perhaps, the best example of the contribution of ASCA to this field is
the case of the SNR G11.2-0.3. Vasisht et al. (Fig. 1;1996 ApJ 456, L59)
used ASCA to detect a plerion in this remnant, which was only hinted
at by previous X-ray observations, and Torii et al. (1997 ApJ 489,
L145) detected a 65 ms pulsar with ASCA, which had not been detected
in the radio.
G11.2-0.3 was suggested to be the remnant of the
historical SN of A.D. 386 (Clark & Stephenson 1977); the rapid
pulsation and the temperature of the shock derived from the ASCA
data are both consistent with a remnant age of ~1600 yr. This is
only the second association of a pulsar with a historical supernova
after the Crab and its pulsar.

The detection of a synchrotron nebula around an as yet undetected
pulsar also lends support to the standard picture of NS birth and SNR
evolution. Harrus et al. (1996 ApJ 464L, 161) detected the X-ray
synchrotron nebula around the known radio pulsar PSR B1853+01 in the
SNR W44. In contrast, Slane et al. (1997 ApJ, 485, 221) report the
detection of a plerion in CTA-1 which was previously unknown in the
radio. Harrus, Hughes, & Slane (1998 ApJ, 499, 273) report the
detection of the synchrotron nebula in MSH11-62 which was known in the
radio. Other remnants for which ASCA has detected a localized
region of non-thermal emission are Kes 75 (Helfand 1994 New Horizons
meeting), G292.0+1.8 (Torii et al. IAU proceedings), G327.1-1.1
(Sun et al. in preparation), and MSH15-56 (Plucinsky et al. 1998 Elba
Proceedings). ASCA also
detected the plerionic component in the SNR
N157B in the LMC which prompted a followup observation by RXTE, which
detected a 16 ms pulsar !

Figure 1. G11.2-0.3 in four energy bands in the adjacent panels.
The upper left is the ASCA band from 0.5 to 3.3 keV, the upper
right is the ASCA band from 3.3 to 9.0 keV, and the lower left is
the ASCA band over the entire band from 0.5 to 9.0 keV. The lower
right panel is the Einstein HRI. The ASCA data demonstrate
clearly the existence of the X-ray plerion. The image is from
Vasisht et al. 1996 (ApJ\ 456, L59).

Non-thermal Emission in SNRs

ASCA has detected localized regions of non-thermal X-ray emission
which cannot be explained by synchrotron emission from a pulsar
nebula. ASCA observations solved the long-standing mystery of the
spectrum of SN1006 by localizing the non-thermal emission to the
bright rims of the remnant (Koyama et al. 1995 Nature 378, 255), which
has been interpreted as synchrotron emission from electrons with
energies up to 100 TeV accelerated in the remnant blast wave (Reynolds
1996 ApJ 459, L13). ASCA discovered an extended region of hard X-ray
emission in the SNR IC 443 (Keohane et al. 1997, ApJ 484,
350). It is coincident with a region of strong
interaction between the remnant shock, and a dense molecular cloud.
The authors speculate that the hard X-ray emission arises from TeV
electrons whose population has been enhanced by virtue of shock-cloud
collisions. If this is correct, then ASCA has unveiled a second
means by which supernova remnants create high energy cosmic rays.

Perhaps the most revolutionary observations will be those of the
enigmatic remnant G347.5-0.5. This remnant was first detected as a
bright source in the ROSAT all-sky survey and was resolved into a
shell-type SNR. The bright NW shell was caught serendipitously in an
ASCA galactic plane survey pointing and the spectrum was revealed
to be non-thermal (Koyama et al. 1997 PASJ 49, L7). ASCA
observations of the entire remnant show that the outer shell and the
interior of the remnant also have a non-thermal spectrum (Slane et al.
1998 ApJ in preparation). There is no evidence of any thermal X-ray
emission from any part of this remnant; this is a puzzling yet
exciting result! The shell-type X-ray morphology, completely
non-thermal spectrum, and relatively large size
are difficult to explain by the
mechanisms observed in other remnants; nevertheless, these data
confirm the power of ASCA's imaging and spectral capabilities.

SNR Surveys

Two different types of surveys have been initiated in the last several
years to utilize ASCA's unique capabilities to provide
moderate-resolution spectra of heavily absorbed objects. First, a
followup of ROSAT all-sky survey sources which are believed to be
extended and which are coincident with known radio SNR has been
started. Three of the first five targets have been detected in the
X-rays with ASCA. G337.2-0.7 exhibited booming lines of Si, S, Ar
and Ca with supersolar abundances indicating that it is possibly a
young, ejecta-dominated remnant. G309.2-0.6 also has strong Si and S
lines in addition to a strong Fe line. G7.7-3.7 has a thermal
spectrum with nearly solar abundances. The second survey project is
aimed at detecting small diameter radio remnants with ASCA. The
first three targets have been observed and two of the remnants have
been detected. G340.6+0.3 is clearly detected and shows line
emission. G328.4-0.2 is detected but shows a complex spectrum with
a hard tail. Both of these surveys have produced promising results in
their first years and should increase the number of known X-ray
remnants.

LMC remnants

ASCA has also made a systematic study of the SNRs in the LMC. One of
the early results of this project was the discovery of new
ejecta-dominated remnants of Type Ia SNe (Hughes et al. 1995 ApJ 444,
L81). This work showed how it was possible to determine the type of
the SN explosion from a comparison of the ASCA X-ray spectra of the
remnant with the nucleosynthetic yields expected from Type Ia and II
SNe. One surprising conclusion was that roughly one-half of the SNRs
produced in the LMC within the last ~1500 yrs came from Type Ia
SNe. The fraction expected based on extragalactic patrols is more like
10%-20%. Hughes, Hayashi, & Koyama (1998 ApJ accepted) used the
X-ray spectral information provided by ASCA in conjunction with a
self-consistent nonequilibrium ionization model assuming a Sedov
solution for the dynamical evolution, to deduce the ages, ambient
interstellar densities, initial explosion energies, and metal
abundances for seven middle-aged remnants. For the remnants for which
the ionization timescale age and the Sedov dynamical age agree the
derived mean explosion energy is 1.1+/-0.5x10^51 ergs, in
excellent agreement with the canonical value. For the remnants N63A,
N132D, and N49B, the ionization timescale ages are significantly less
than the Sedov dynamical ages and the explosion energies are rather
large. Hughes, Hayashi, & Koyama suggest that both of these
discrepancies can be resolved by invoking a scenario in which the
progenitor was a massive star which had blown out a cavity. They have
also provided a new and independent determination of the gas phase
abundances in the LMC by using the X-ray spectra to determine the
abundances of the astrophysically common elements O, Ne, Mg, Si, S,
and Fe, to be 0.2-0.4 times solar. The X-ray-derived values are
consistent with those from optical studies (e.g. Russell & Dopita
1992 ApJ 384, 508), but the X-ray data provide significantly more
accurate measurements of the important species Mg and Si (for which
few good emission lines in the optical band exist). Since the ISM
contains the integrated sum of material lost by stars in winds and SNe
over the galaxy's life, the chemical composition is one of the
principle probes of the galaxy's star formation history.

Thermal X-ray Emission

The spectral capability of the SIS has been used to perform detailed
modeling of the spectra of young Galactic remnants, and thereby learn
new insights about the origin of the X-ray emission. Borkowski et al.
(1996, ApJ 466, 866) performed a careful study of the Fe K lines from
the core-collapse remnant Cas A, and concluded that its strength is
accounted for only if a substantial amount of interstellar dust is
present. In contrast, when Hwang, Hughes, & Petre (1998, ApJ 497, 833)
performed the same analysis on the spectrum of the Type Ia remnant
Tycho, they were able to place severe constraints on the amount of dust
present. They also find that multiple emission components, presumably
from ejecta and the blast wave, are required to explain the relative
strengths of the Fe K and L lines. Hwang & Gotthelf (1997, ApJ 475,
665) produced a set of spatially filtered, narrow band maps of Tycho.
Although each map has an overall morphology similar to the broad band
map, each shows a set of distinctive features. Overall they find the
emission morphology is consistent with a spherical shell, and not with
a torus, and that some radial mixing of ejecta has occurred. Vink,
Kastra & Bleeker (1997, A&A 328, 628) find a dramatic temperature
gradient across RCW 86. They also find a relative lack of line
emission which they suggest is the result of an electron distribution
with a supra-thermal tail.

Discovery of Young X-ray Pulsars with ASCA

The study of pulsars in SNRs is critical to our understanding of the
evolution of young neutron stars. It allows us to probe these
fascinating objects for which only an astrophysical laboratory is
available.

For example, ASCA has nearly doubled the number of known Crab-like
pulsars with the discovery of a 65 ms pulsar in the young plerionic
SNR G11.2-0.3 (Torii et al. 1997 ApJ 489, L145) and the 16 ms pulsar
LMC SNR N157B (Marshall et al. 1998 ApJ 499, L179). The latter pulsar
is located near the famous 50 ms LMC pulsar and is the most rapidly
rotating pulsar associated with a SNR yet discovered. The properties
of these pulsars are consistent with the canonical picture of a young
pulsar born as a rapidly rotating (~10 ms) NS powered by the
spin-down energy of a magnetized-dipole (~ 10^12 G).

Several other pulsars detected by ASCA are considered candidate SNR
pulsars, due to their properties and proximity to young SNRs. These
include the 69 ms pulsar discovered near the SNR RCW 103 (Torii et al.
1998 ApJ 494, L207) and the X-ray emission from the 63 ms radio
pulsar PSR J1105-6107 (Gotthelf & Kaspi 1998 ApJ 497, L29). The
elusive NS candidate in the center of RCW 103, re-discovered by ASCA,
may well be a pulsar with unseen pulses due to unfavorable beaming
geometry (Fig. 2; Gotthelf et al. 1997 ApJ 497, L29).

Figure 2. The ASCA SIS image of the SNR RCW 103 in two spectral bands.
Below 1.5 keV (Bottom) the flux from RCW 103 is predominately from the
soft thermal emission of a shocked plasma, typical of a young
SNR. Above 3 keV (Top) the intriguing central point source in
RCW 103, 1E161348-505, is evident un-obscured by the nebula emission.
Due North of the central source is the serendipitous ASCA point source,
the 69 ms pulsar PSR J1617-5055. The images were produced using data
from both SIS cameras and have been exposure corrected and smoothed.

ASCA is also revealing a new class of slowly rotating NS candidates
associated with SNR. These have profound implications for the theory
of NS evolution. Perhaps the best example is the discovery using
ASCA of 12
sec pulsations from the central object in the young SNR Kes 73
(Vasisht & Gotthelf 1997 ApJ 486, L129). Despite numerous previous
observations, this pulsar had eluded detection by the Einstein and
ROSAT observatories, which lacked the broad spectral band imaging
capabilities of ASCA.

ASCA also discovered AX 1845-0258, a highly absorbed 7 sec pulsar in
the distant Milky Way (Gotthelf & Vasisht 1998 NA 3, L293) and the 11
sec pulsar located in Scorpio (Sugizaki et al. 1998
PASJ 49, L25). The characteristics of these pulsars are similar to
those of the ``anomalous X-ray pulsars'' (Mereghetti & Stella 1995
ApJ 442, L17; van Paradijs et al. 1995 A&A 299, L41), personified by
the well studied 7 sec pulsar in CTB 109 (Gregory & Fahlman 1980;
Corbet et al. 1995 ApJ 433, 786). The spin periods for these objects
lie in the range of 5-12 sec and their ASCA spectra are unusually
steep ( 0.6 keV or Gamma > 3) for an rotation- or accretion-
powered pulsar. Their luminosities are typically around ~10^35 ergs/s
and seem to be steady over many years. An accretion
origin is unlikely as they lack an observed counterpart, show no
indication of binary motion, or display flux variability as is typical
of accreting systems.

ASCA is playing a key role in increasing our understanding of the
evolution of young NSs. By the detection of new anomalous X-ray
pulsars, and subsequent monitoring of their pulse and flux histories,
ASCA has shown that the standard paradigms of young pulsar evolution
may no longer be valid. For example, a follow-up ASCA observation of
Kes 73 confirms the remarkable spin-down rate of its pulsar and that
the measured luminosity cannot be simply powered by radiative losses
due to spin-down (Gotthelf et al. 1998 in preparation). The inferred
magnetic field for a rotating magnetic dipole is well above the
quantum limit of 4x10^14 G. The Kes 73 pulsar is likely the
first example of a ``magnetar'' (Thompson & Duncan 1995 MNRAS 275,
255), a NS with an enormous magnetic field. The pulsar was likely spun
down rapidly by magnetic field decay or possibly born as a slow
pulsar. In either case, the ASCA data require us to consider
alternative NS evolution scenarios in direct competition with the
standard theory.